
The study of gravitational wave sources: The discovery of electro-magnetic emission following the detection of gravitational waves (GW) from the mergers of binaries involving neutron stars[1]}, [2]}. Such detections will be the key to using these events for addressing fundamental physics questions, such as the origin of the heaviest elements and the expansion rate of the universe[3]}. For GW triggers, ULTRASAT's wide field-of-view amply covers the angular error regions expected to be provided by GW detectors starting 2025[4]}. It will provide continuous UV light curves as well as early alerts that will enable ground-based follow-up spectroscopy and monitoring of optical and infrared emission predicted to arise later.

Early detection of Supernovae: ULTRASAT will collect early UV light curves of hundreds of core-collapse supernovae (CC-SNe) to measure the radii and surface composition of their massive progenitors, as well as to determine explosion parameters[5]}. Connecting the pre-explosion stars with their diverse explosive output will chart how the population of massive stars impact their environment through mass loss and explosion, and will specify initial conditions for explosion models[6]}, [7]}, [8]}. Mass loss tracers will further constrain pre-explosion evolution, allowing comprehensive investigation of the final evolution and explosive death of massive stars. During its 3 years mission, ULTRASAT is expected to detect \(>40\)  CC-SNe during the shock breakout phase, and \(>500\)  CC-SNe during the shock cooling phase.

Determine the high energy flare frequency distribution of stars and guide future exoplanet atmospheric studies: ULTRASAT will monitor \(\sim 10^6\)  stars. The long baseline high cadence survey strategy will allow us to characterize UV flares and determine the low frequency - high energy tail of the flaring distribution function, specifically from dM stars, attractive exoplanet hosts for atmospheric characterization due to favorable radii ratio, known to be prodigiously UV flaring stellar objects [9]}, [10]}. ULTRASAT will measure, for the first time, the NUV flare frequency and luminosity distribution for stars as functions of both spectral subclass, and stellar rotation period. This will allow the community to asses which stellar types are favored as habitable planet hosts and guide future spectroscopic bio-marker searches with, e.g., JWST and the upcoming ELTs [11]}, [12]}.

Active galactic nuclei: ULTRASAT is poised to provide novel insights into the central engines of Active Galactic Nuclei (AGN), which are powered by accreting supermassive black holes (SMBHs), as the emission from these systems peaks in the UV and is known to vary on essentially all timescales. The study of AGN-related phenomena with ULTRASAT is expected to be broadly split into two regimes. First, ULTRASAT will monitor samples of persistent, vigorously accreting SMBHs over unprecedently short timescales, much shorter than the dynamical timescale in the innermost parts of their accretion disks[13]}. This will advance our understanding of persistent accretion disks[14]}, and may provide new ways to quantify key SMBH properties, such as mass, growth rate, and even spin[15]}. Second, ULTRASAT will discover and survey SMBH-related, UV-bright transient phenomena, marking extreme changes to the accretion flows and allowing us to see how SMBHs "turn on" or "off" on exceedingly short timescales[16]}, [17]}, and to obtain insights regarding super-Eddington accretion[18]}, [19]}.

Tidal Disruption Events: Stars torn apart in tidal disruption events (TDEs) emit luminous ultraviolet flares[20]}, [21]}, [22]}. ULTRASAT will be able to detect hundreds to thousands of events per year (of which \(\sim 50\)  will be at optical magnitudes brighter than 19 and thus easy to follow up from the ground). These discoveries will allow us to constrain the currently debated emission mechanisms of TDEs [23]}, measure their rates as a function of redshift [24]} and probe their peculiar host galaxy preferences [21]}, [26]}.

